Magnetostriction is a phenomenon that occurs in magnetic materials such as ferrites, metals and alloys. Magnetization of the materials causes a change dl in a dimension 1, creating a strain or magnetostriction .lambda. represented by .lambda.=dl/1. Magnetostriction is a material constant that can be either negative or positive. A magnetic material exists in one of three states or regimes. Above a Curie temperature, the material is in a paramagnetic regime and exhibits no magnetization. Below the Curie temperature, the material may be in either a ferromagnetic regime or a saturation magnetization regime. In the ferromagnetic regime, spontaneous magnetization occurs in small, randomly ordered molecular magnetic domains throughout the material. The overall magnetization of the material is, however, zero. A strong magnetic field, sufficient to align all the molecular magnetic domains, may be applied to the material to place it in a saturation magnetization regime. In this regime, the alignment of the molecular magnetic domains produces a maximum length change in the material and provides a value for the saturation magnetostriction of the material.
Power modules are employed in many electronic devices to power the components therein. Power modules were initially available in through-hole packages consisting of a metal or plastic case, housing a printed wiring board (PWB) on which power module components were mounted.
Electronic components are currently migrating towards surface-mount packaging in overwhelming proportions. Board-mounted power modules will inevitably follow, if only to assure assembly compatibility with this packaging technology. Surface-mount assembly operations, however, typically involve severe reflow temperatures and wash (or cleaning) cycles that may damage components in the power modules. As a result, power module circuits are encapsulated in a rigid epoxy molding compound via, in most cases, a transfer molding process. As a dense glass-filled epoxy with a high glass transition temperature and a high modulus, the molding compound is capable of withstanding the high temperatures found in surface-mount assembly operations. During encapsulation, the molding compound completely fills around all the components in the power module circuits, creating a solid package for the power module and providing a good thermal path for heat generating components. The molding compound thus protects the power module circuits from surface-mount assembly operations.
The protection provided by the molding compound comes, however, at a cost. As the molding compound cools from a molding temperature to room temperature, it shrinks. Substantial thermal shrinkage stresses are thus imposed on the components in the power modules by the high modulus molding compound.
Power modules typically use ferrite materials (e.g., manganese zinc (MnZn)) ! as core materials in magnetic devices such as power transformers and energy storage inductors. As the molding compound shrinks and thermal shrinkage stresses are imposed on the ferrite materials, large strains are created, restricting the movements of the small, molecular magnetic domains during external magnetic field excitation. The required degree of alignment of the molecular magnetic domains cannot be achieved.
Strain pinning between the domain walls occurs, increasing dissipation in the ferrite materials. The ferrite materials, therefore, cannot fully enter the saturation regime. A pronounced decrease in the magnetic properties results, with a corresponding degradation in performance of the magnetic devices (e.g., power transformers and energy storage inductors). As described in U.S. Ser. No. 08/604,637 filed on Feb. 21, 1996, "Encapsulated Package for Power Magnetic Devices and Method of Manufacture Therefor," magnetic devices containing ferrite materials, therefore, must be protected from the thermal shrinkage stresses of the molding compound to retain full functionality in surface-mount power modules or power modules in general.
Since the transfer molding process is widely used in packaging integrated circuits, a determination of molding stresses in the packages during molding is necessary to the design of a long-life and robust package. Precise knowledge of thermal shrinkage stresses is also necessary in the development of protection schemes for the ferrite materials used in power modules.
Knowledge of thermal shrinkage stresses in a molded package is typically obtained through analytical models and determined by conventional stress analysis or finite element analysis. This knowledge, however, is only theoretical. It would be advantageous therefore, to measure the shrinkage stresses during the three stages of molding (i.e., filling, packing and cooling).
Accordingly, what is needed in the art is a method of empirically determining the stresses (including shrinkage stresses) present in a molded package.